Cardiac arrhythmia is a group of medical conditions, in which the electrical activity of the heart is irregular, or is slower or faster than normal. Some arrhythmias are life-threatening, and can cause cardiac arrest or sudden death. Others cause, or predispose to, other aggravating symptoms or disease, including stroke. Fibrillation is a serious form of arrhythmia, in which the heart muscle presents with irregular or quivering motion due to lack of unity in the function of contractile cells. Fibrillation can affect the atrium (Atrial Fibrillation (AF) or Atrial Flutter (AFl)), or the ventricle (Ventricular Fibrillation (VF)).
Atrial fibrillation (AF) is an abnormal heart rhythm (cardiac arrhythmia) which involves the two small, upper heart chambers (the atria). Heart beats in a normal heart begin after electricity generated in the atria by the sinoatrial node spreads through the heart and causes contraction of the heart muscle and pumping of blood. In AF, the regular electrical impulses of the sinoatrial node are replaced by disorganized, rapid electrical impulses which result in irregular heart beats.
Atrial fibrillation is the most common cardiac arrhythmia. The risk of developing atrial fibrillation increases with age—AF affects four percent of individuals in their 80s. An individual may spontaneously alternate between AF and a normal rhythm (paroxysmal atrial fibrillation) or may continue with AF as the dominant cardiac rhythm without reversion to the normal rhythm (chronic atrial fibrillation). Atrial fibrillation is often asymptomatic, but may result in symptoms of palpitations, fainting, chest pain, or even heart failure. These symptoms are especially common when atrial fibrillation results in a heart rate which is either too fast or too slow. In addition, the erratic motion of the atria leads to blood stagnation (stasis) which increases the risk of blood clots that may travel from the heart to the brain and other areas. Thus, AF is an important risk factor for stroke, the most feared complication of atrial fibrillation.
The symptoms of atrial fibrillation may be treated with medications which slow the heart rate. Several medications as well as electrical cardioversion may be used to convert AF to a normal heart rhythm. Surgical and catheter-based therapies may also be used to prevent atrial fibrillation in certain individuals. People with AF are often given blood thinners such as warfarin to protect them from strokes.
Any patient with 2 or more identified episodes of atrial fibrillation is said to have recurrent atrial fibrillation. This is further classified into paroxysmal and persistent based on when the episode terminates without therapy. Atrial fibrillation is said to be paroxysmal when it terminates spontaneously within 7 days, most commonly within 24 hours. Persistent or chronic atrial fibrillation is AF established for more than seven days. Differentiation of paroxysmal from chronic or established AF is based on the history of recurrent episodes and the duration of the current episode of AF (Levy S., J Cardiovasc Electrophysiol. 8 Suppl, S78-82 (1998)).
Lone atrial fibrillation (LAF) is defined as atrial fibrillation in the absence of clinical or echocardiographic findings of cardiopulmonary disease.
Atrial fibrillation is usually accompanied by symptoms related to either the rapid heart rate or embolization. Rapid and irregular heart rates may be perceived as palpitations, exercise intolerance, and occasionally produce angina and congestive symptoms of shortness of breath or edema. Sometimes the arrhythmia will be identified with the onset of a stroke or a transient ischemic attack (TIA). It is not uncommon to identify atrial fibrillation on a routine physical examination or electrocardiogram (ECG/EKG), as it may be asymptomatic in some cases. Paroxysmal atrial fibrillation is the episodic occurrence of the arrhythmia and may be difficult to diagnose. Episodes may occur with sleep or with exercise, and their episodic nature may require prolonged ECG monitoring (e.g. a Holter monitor) for diagnosis.
Atrial fibrillation is diagnosed on an electrocardiogram, an investigation performed routinely whenever irregular heart beat is suspected. Characteristic findings include absence of P waves, unorganized electrical activity in their place and irregularity of R-R interval due to irregular conduction of impulses to the ventricles. If paroxysmal AF is suspected, episodes may be documented with the use of Holter monitoring (continuous ECG recording for 24 hours or longer).
While many cases of AF have no definite cause, it may be the result of various other problems (see below). Hence, renal function and electrolytes are routinely determined, as well as thyroid-stimulating hormone and a blood count. A chest X-ray is generally performed. In acute-onset AF associated with chest pain, cardiac troponins or other markers of damage to the heart muscle may be ordered. Coagulation studies (INR/aPTT) are usually performed, as anticoagulant medication may be commenced. A transesophageal echocardiogram may be indicated to identify any intracardiac thrombus (Fuster V., et al., Circulation; 104, 2118-2150 (2001)).
Atrial Flutter (AFl) is characterized by an abnormal fast heart rhythm in the atria. Patients who present with atrial flutter commonly also experience Atrial Fibrillation and vice versa (Waldo, A., Progr Cardiovasc Disease, 48:41-56 (2005)). Mechanistically and biologically, AF and AFl are thus likely to be highly related.
AF (and AFl) is linked to several cardiac causes, but may occur in otherwise normal hearts. Known associations include: High blood pressure, Mitral stenosis (e.g. due to rheumatic heart disease or mitral valve prolapse), Mitral regurgitation, Heart surgery, Coronary artery disease, Hypertrophic cardiomyopathy, Excessive alcohol consumption (“binge drinking” or “holiday heart”), Hyperthyroidism, Hyperstimulation of the vagus nerve, usually by having large meals (“binge eating”), Lung pathology (such as pneumonia, lung cancer, pulmonary embolism, Sarcoidosis), Pericarditis, Intense emotional turmoil, and Congenital heart disease.
The normal electrical conduction system of the heart allows the impulse that is generated by the sinoatrial node (SA node) of the heart to be propagated to and stimulate the myocardium (muscle of the heart). When the myocardium is stimulated, it contracts. It is the ordered stimulation of the myocardium that allows efficient contraction of the heart, thereby allowing blood to be pumped to the body. In atrial fibrillation, the regular impulses produced by the sinus node to provide rhythmic contraction of the heart are overwhelmed by the rapid randomly generated discharges produced by larger areas of atrial tissue. An organized electrical impulse in the atrium produces atrial contraction; the lack of such an impulse, as in atrial fibrillation, produces stagnant blood flow, especially in the atrial appendage and predisposes to clotting. The dislodgement of a clot from the atrium results in an embolus, and the damage produced is related to where the circulation takes it. An embolus to the brain produces the most feared complication of atrial fibrillation, stroke, while an embolus may also lodge in the mesenteric circulation (the circulation supplying the abdominal organs) or digit, producing organ-specific damage.
Treatment of atrial fibrillation is directed by two main objectives: (i) prevent temporary circulatory instability; (ii) prevent stroke. The most common methods for achieving the former includes rate and rhythm control, while anticoagulation is usually the desired method for the latter (Prystowsky E. N., Am J Cardiol.; 85, 3D-11D (2000); van Walraven C, et al., Jama. 288, 2441-2448 (2002)). Common methods for rate control, i.e. for reducing heart rate to normal, include beta blockers (e.g., metotprolol), cardiac glycosides (e.g., digoxin) and calcium channel blockers (e.g., verapamil). All these medications work by slowing down the generation of pulses from the atria, and the conduction from the atria to the ventricles. Other drugs commonly used include quinidine, flecainide, propafenone, disopyramide, sotalol and amiodarone. Rhythm control can be achieved by electrical cardioversion, i.e. by applying DC electrical shock, or by chemical cardioversion, using drugs such as amiodarione, propafenone and flecamide.
Preventive measures for stroke include anticoagulants. Representative examples of anticoagulant agents are Dalteparin (e.g., Fragmin), Danaparoid (e.g., Orgaran), Enoxaparin (e.g., Lovenox), Heparin (various), Tinzaparin (e.g., Innohep), Warfarin (e.g., Coumadin). Some patients with lone atrial fibrillation are sometimes treated with aspirin or clopidogrel. There is evidence that aspirin and clopidogrel are effective when used together, but the combination is still inferior to warfarin (Connolly S., et al. Lancet; 367, 1903-1912 (2006)). (2) The new anticoagulant ximelagatran has been shown to prevent stroke with equal efficacy as warfarin, without the difficult monitoring process associated with warfarin and with possibly fewer adverse haemorrhagic events. Unfortunately, ximegalatran and other similar anticoagulant drugs (commonly referred to as direct thrombin inhibitors), have yet to be widely licensed.
Determining who should and should not receive anti-coagulation with warfarin is not straightforward. The CHADS2 score is the best validated method of determining risk of stroke (and therefore who should be anticoagulated). The UK NICE guidelines have instead opted for an algorithm approach. The underlying problem is that if a patient has a yearly risk of stroke that is less than 2%, then the risks associated with taking warfarin outweigh the risk of getting a stroke (Gage B. F. et al. Stroke 29, 1083-1091 (1998))
Atrial fibrillation can sometimes be controlled with treatment. The natural tendency of atrial fibrillation, however, is to become a chronic condition. Chronic AF leads to an increased risk of death. Patients with atrial fibrillation are at significantly increased chance of stroke.
Atrial fibrillation is common among older adults. In developed countries, the number of patients with atrial fibrillation is likely to increase during the next 50 years, due to the growing proportion of elderly individuals (Go A. S. et al., Jama., 285, 2370-2375 (2001))(3). In the Framingham study the lifetime risk for development of AF is 1 in 4 for men and women 40 years of age and older. Lifetime risks for AF are high (1 in 6). According to data from the National Hospital Discharge Survey (1996-2001) on cases that included AF as a primary discharge diagnosis found that 45% of the patients are male, and that the mean age for men was 66.8 years and 74.6 for women. The racial breakdown for admissions was found to be 71.2% white, 5.6% black, 2% other races, and 20% not specified. Furthermore, African American patients were, on average, much younger than other races. The incidence in men ranged from 20.58/100,000 persons per year for patients ages 15-44 years to 1203/100,000 persons per years for those ages 85 and older. From 1996-2001, hospitalizations with AF as the first listed diagnosis, increased by 34%.
Stroke is a common and serious disease. Each year in the United States more than 600,000 individuals suffer a stroke and more than 160,000 die from stroke-related causes (Sacco, R. L. et al., Stroke 28, 1507-17 (1997)). Furthermore, over 300,000 individuals present with Transient Ischemic Attack, a mild form of stroke, every year in the US. In western countries stroke is the leading cause of severe disability and the third leading cause of death (Bonita, R., Lancet 339, 342-4 (1992)). The lifetime risk of those who reach the age of 40 exceeds 100%.
The clinical phenotype of stroke is complex but is broadly divided into ischemic (accounting for 80-90%) and hemorrhagic stroke (10-20%) (Caplan, L. R. Caplan's Stroke: A Clinical Approach, 1-556 (Butterworth-Heinemann, 2000)). Ischemic stroke is further subdivided into large vessel occlusive disease (referred to here as carotid stroke), usually due to atherosclerotic involvement of the common and internal carotid arteries, small vessel occlusive disease, thought to be a non-atherosclerotic narrowing of small end-arteries within the brain, and cardiogenic stroke due to blood clots arising from the heart usually on the background of atrial fibrillation or ischemic (atherosclerotic) heart disease (Adams, H. P., Jr. et al., Stroke 24, 35-41 (1993)). Therefore, it appears that stroke is not one disease but a heterogeneous group of disorders reflecting differences in the pathogenic mechanisms (Alberts, M. J. Genetics of Cerebrovascular Disease, 386 (Futura Publishing Company, Inc., New York, 1999); Hassan, A. & Markus, H. S. Brain 123, 1784-812 (2000)). However, all forms of stroke share risk factors such as hypertension, diabetes, hyperlipidemia, and smoking (Sacco, R. L. et al., Stroke 28, 1507-17 (1997); Leys, D. et al., J. Neurol. 249, 507-17 (2002)). Family history of stroke is also an independent risk factor suggesting the existence of genetic factors that may interact with environmental factors (Hassan, A. & Markus, H. S. Brain 123, 1784-812 (2000); Brass, L. M. & Alberts, M. J. Baillieres Clin. Neurol. 4, 221-45 (1995)).
The genetic determinants of the common forms of stroke are still largely unknown. There are examples of mutations in specific genes that cause rare Mendelian forms of stroke such as the Notch3 gene in CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarctions and leukoencephalopathy) (Tournier-Lasserve, E. et al., Nat. Genet. 3, 256-9 (1993); Joutel, A. et al., Nature 383, 707-10 (1996)), Cystatin C in the Icelandic type of hereditary cerebral hemorrhage with amyloidosis (Palsdottir, A. et al., Lancet 2, 603-4 (1988)), APP in the Dutch type of hereditary cerebral hemorrhage (Levy, E. et al., Science 248, 1124-6 (1990)) and the KRIT1 gene in patients with hereditary cavernous angioma (Gunel, M. et al., Proc. Natl. Acad. Sci. USA 92, 6620-4 (1995); Sahoo, T. et al., Hum. Mol. Genet. 8, 2325-33 (1999)). None of these rare forms of stroke occur on the background of atherosclerosis, and therefore, the corresponding genes are not likely to play roles in the common forms of stroke which most often occur with atherosclerosis.
It is very important for the health care system to develop strategies to prevent stroke. Once a stroke happens, irreversible cell death occurs in a significant portion of the brain supplied by the blood vessel affected by the stroke. Unfortunately, the neurons that die cannot be revived or replaced from a stem cell population. Therefore, there is a need to prevent strokes from happening in the first place. Although we already know of certain clinical risk factors that increase stroke risk (listed above), there is an unmet medical need to define the genetic factors involved in stroke to more precisely define stroke risk. Further, if predisposing alleles are common in the general population and the specificity of predicting a disease based on their presence is low, additional loci such as protective loci are needed for meaningful prediction of disposition of the disease state. There is also a great need for therapeutic agents for preventing the first stroke or further strokes in individuals who have suffered a previous stroke or transient ischemic attack.
AF is an independent risk factor for stroke, increasing risk about 5-fold. The risk for stroke attributable to AF increases with age. AF is responsible for about 15-20% of all strokes. AF is also an independent risk factor for stroke recurrence and stroke severity. A recent report showed people who had AF and were not treated with anticoagulants had a 2.1-fold increase in risk for recurrent stroke and a 2.4 fold increase in risk for recurrent severe stroke. People who have stroke caused by AF have been reported as 2.23 times more likely to be bedridden compared to those who have strokes from other causes.
There is a need for an understanding of the susceptibility factors leading to increased predisposition for AF and stroke. Identification of at-risk variants for AF can, for example, be useful for assessing which individuals are at particularly high risk for AF and subsequent stroke. Furthermore, preventive treatment can be administered to individuals suffering from AF and who are carriers of at-risk susceptibility variants for AF and/or stroke. Finally, identification of at-risk variants for AF and/or stroke can lead to the identification of new targets for drug therapy, as well as the development of novel therapeutic measures.